The fluid separation apparatus for separating components of fluid by using a membrane having a selective permeability is applied to various techniques such as gas permeation, liquid permeation, dialysis, ultrafiltration, reverse osmosis, or the like. Recently, attention has been particularly given to the reverse osmosis which is especially effective for desalination and purification of sea water or brackish water, for recovering useful or harmful components from waste water or for reuse of water. Membrane separation apparatuses are employed for carrying out these techniques and are classified into flat membrane type, tubular type, spiral type and hollow-fiber type according to the shape and form of the semi-permeable membrane used therein. Among these, a hollow-fiber type apparatus has very high membrane separation efficiency per unit volume of the apparatus because no substrate for the semi-permeable membrane is needed. Hollow-fiber membranes are particularly suited for reverse osmosis separations.
The reverse osmosis is usually carried out by treating a fluid under pressure higher than the osmotic pressure of the fluid, by which the components of fluid are separated via a membrane having a selective permeability. The pressure may vary with the kinds of fluids to be treated, the properties of the selectively permeable membranes, or the like, but is usually in the range from 40 to 2,000 psi for hollow-fiber membranes. Accordingly, it is very important that the membrane, the supporting material and the casing vessel used for the reverse osmosis should have a high pressure resistance.
There is a tendency in the fluid separation industry to enlarge an apparatus to be used in the membrane separation techniques in order to adapt it for industrial performance. Particularly, enlargement of a reverse osmosis hollow-fiber apparatus is of great advantage to adapt it for industrial performance because the apparatus has high volumetric efficiency. The larger-size apparatus can feed a much larger amount of fluid than that of a conventional apparatus. In comparison with a conventional apparatus, a large-size fluid separation apparatus has many advantages including: (a) the cost per unit volume of the apparatus can be reduced; (b) the steps of manufacturing process of a hollow-fiber assembly can be reduced; (c) and the number of external pipings, valves and instruments equipped around the apparatus can be reduced. Accordingly, in the case of carrying out a large-scale membrane separation treatment, a large-sized apparatus, which needs fewer parts than those of a conventional apparatus, is desired.
Two conventional methods are known to enlarge a hollow-fiber membrane separation apparatus. One is elongation of the apparatus. The other is enlargement of the diameter of the apparatus. However, when the apparatus is elongated, fluid to be treated is difficult to flow through inside of the hollow fiber because the distance between the opening ends of the hollow fiber become long and pressure loss of the fluid is increased. This results in polarization down the length of each hollow fiber, ultimately resulting in non-uniform flow of the permeate through each hollow-fiber membrane. For example, in the case of a single-ended hollow-fiber bundle in which one end of the bundle is plugged, the fluid pressure inside each hollow fiber may be an order of magnitude greater at the sealed end versus the open end. Consequently, the rate of permeate flow through the hollow-fiber membrane increases from the sealed end to the open end. The percentage of solute rejection will typically increase from the sealed end to the open end of each hollow fiber. The ultimate impact is non-uniform flow of permeate, poor non-uniform salt rejection and a greater tendency to foul the apparatus. This problem is magnified in an apparatus with a reverse osmosis membrane of hollow fiber. Such a fluid separation device has a greater pressure resistance in comparison with other types of membranes such as flat-membrane type, tubular type or spiral type.
On the other hand, when the diameter of the apparatus is enlarged, cost of the pressure vessel increases rapidly. Moreover, depending on the geometry of the apparatus, the enlarged diameter may result in increases in the pressure drop of the reject fluid flowing radially through the bundles. As a result, the reject flow rate of the fluid through the device must be increased which results in having to operate at lower conversion and which ultimately increases the opening costs. The enlarged diameter of the apparatus may also cause a concentration polarization phenomenon at the areas where the flow rate of the fluid to be treated is small or the fluid stays, because the flow of the fluid through and across the hollow-fiber layer surface becomes uneven, between the inner portion and the outer portion in the layer.
At the same time, it is economical to enlarge the ratio of length/radius of the cylindrical pressure vessel which contains the membrane assemblies, in view of scale-up. In other words, it is preferable to enlarge the apparatus in the longitudinal direction.
The prior art describes hollow-fiber membrane-type fluid separation apparatuses where at least one pair of hollow-fiber assemblies are contained. The configuration of the hollow-fiber bundles facilitates the separation of large volumes of liquid by arranging the hollow-fiber assemblies in series. A pair of permeate fluid pipes may be arranged internally in order to pass the concentrated permeate fluid from one hollow-fiber bundle directly into the second hollow-fiber bundle for further separation by reverse osmosis. However, these devices often require complicated hardware and multiple pressure compartments within the pressure vessel. Also, the fluid to be treated is separated by bundles in series thereby reducing the volumetric efficiency of the apparatus.
The prior art also describes a hollow-fiber membrane separation apparatus which comprises independent pressurized compartments within the pressure vessel, each containing one bundle of hollow-fiber membranes. The independent pressurized compartments are connected by a series of passages. Such independent pressurized compartments are costly to construct and are prone to failure.
The present invention provides a large-sized hollow-fiber membrane separation apparatus in which a plurality of shorter hollow-fiber bundles are confined within one pressurized compartment and which keeps the advantages of the prior art, with minimum permeate pressure loss down the bore of the hollow fibers and minimum concentration polarization phenomenon. The inventive fluid separation apparatus is a simple, economical device which may be readily adapted from an existing single-bundle fluid-separation apparatus. The present invention also facilitates so-called "inside out" flow of the fluid to be separated. These objectives, as well as other objects and advantages of the present invention, will become apparent to those skilled in the art from the following description.